Cripple Creek and other alkaline-related gold deposits in the southern Rocky Mountains, USA: influence of regional tectonics

Alkaline-related epithermal vein, breccia, disseminated, skarn, and porphyry gold deposits form a belt in the southern Rocky Mountains along the eastern edge of the North American Cordillera. Alkaline igneous rocks and associated hydrothermal deposits formed at two times. The first was during the Laramide orogeny (about 70-40 Ma), with deposits restricted spatially to the Colorado mineral belt (CMB). Other alkaline igneous rocks and associated gold deposits formed later, during the transition from a compressional to an extensional regime (about 35-27 Ma). These younger rocks and associated deposits are more widespread, following the Rocky Mountain front southward, from Cripple Creek in Colorado through New Mexico. All of these deposits are on the eastern margin of the Cordillera, with voluminous calc-alkaline rocks to the west. The largest deposits in the belt include Cripple Creek and those in the CMB. The most important factor in the formation of all of the gold deposits was the near-surface emplacement of relatively oxidized volatile-rich alkaline magmas. Strontium and lead isotope compositions suggest that the source of the magmas was subduction-modified subcontinental lithosphere. However, Cripple Creek alkaline rocks and older Laramide alkaline rocks in the CMB that were emplaced through hydrously altered LREE-enriched rocks of the Colorado (Yavapai) province have ²⁰⁸Pb/²⁰⁴Pb ratios that suggest these magmas assimilated and mixed with significant amounts of lower crust. The anomalously hot, thick, and light crust beneath Colorado may have been a catalyst for large-scale transfer of volatiles and crustal melting. Increased dissolved H₂O (and CO₂, F, Cl) of these magmas may have resulted in more productive gold deposits due to more efficient magmatic-hydrothermal systems. High volatile contents may also have promoted Te and V enrichment, explaining the presence of fluorite, roscoelite (vanadium-rich mica) and tellurides in the CMB deposits and Cripple Creek as opposed to deposits to the south. Deep-seated structures of regional extent that formed during the Proterozoic allowed the magmas to rise to shallow crustal levels. Proterozoic sites of intrusions at 1.65, 1.4, and 1.1 Ga were also important precursors to alkaline-related gold deposits. Many of the larger gold deposits are located at sites of Proterozoic intrusions, and are localized at the intersection of northeast-trending ductile shear zones formed during Mesoproterozoic deformation, and an important north-trending fault formed during 1.1 Ga rifting.     

Ludlow (Silurian) stromatoporoid biostromes from Gotland, Sweden: facies, depositional models and modern analogues

Stacked stromatoporoid‐dominated biostromes of the Ludlow‐age Hemse Group (Silurian) in eastern Gotland, Sweden, are 0·5–5 m thick and a few tens of metres to >1 km in lateral extent. They form one of the world's richest Palaeozoic stromatoporoid deposits. This study compiles published and new data to provide an overall facies model for these biostromes, which is assessed in relation to possible modern analogues. Some biostromes have predominantly in‐place fossils and are regarded as reefs, but lack rigid frameworks because of abundant low‐profile non‐framebuilding stromatoporoids; other biostromes consist of stromatoporoid‐rich rudstones interpreted here as storm deposits. Variation between these two `end‐members' occurs both between interlayered biostromes and also vertically and laterally within individual biostromes. Such variation produces problems of applying established reef classification terms and demonstrates the need for the development of terminology that recognizes taphonomic destruction of reef fabrics. An approach to such terminology is found in all four categories of a recent biostrome classification scheme that are easily recognized in the Hemse biostrome facies: autobiostromes (>60% in place); autoparabiostromes (a mixture of in‐place and overturned reef‐building organisms, 20–60% in place); parabiostromes (builders are overturned and damaged, <20% in place); and allobiostromes (transported and detrital reef material, nothing in place). These categories provide a broad taphofacies scheme for the Hemse biostromes, which are mostly autoparabiostrome to allobiostrome. The biostromes developed on crinoidal grainstone sheets and expanded laterally across relatively flat substrates in a marine setting of low siliciclastic input. Planar erosion surfaces commonly terminate biostrome tops. Three broadly similar modern analogues are identified, each of which has elements in common with the Hemse biostromes, but none of which is an exact equivalent: (a) laterally expanded and coalesced back‐barrier patch reefs behind the Belize barrier, an area influenced by limited accommodation space; (b) a hurricane‐influenced shelf, interpreted for Grand Cayman, where reef cores consist of rubble and lack substantial framework; the wide distribution of rounded pebbles and cobbles of stromatoporoids in the Hemse biostromes most probably resulted from hurricanes; (c) coral carpets in 5–15 m water depth of the northern Red Sea, where lateral expansion of low‐diversity frames dominated by Porites coral has produced low‐profile biostromes up to 8 m thick and several km long. Such carpets accumulated large amounts of carbonate, with little export, as in the Hemse biostromes, although the latter did not build frameworks because of the nature of growth of the stromatoporoids. The notable lack of algae in the Hemse biostrome facies is also a feature of Red Sea coral carpets; nevertheless, coral carpets are ecologically different. Hemse biostromes lack evidence of a barrier reef system, although this may not be exposed; the facies assemblage is consistent with either a storm/hurricane‐influenced mid‐ to upper ramp or back‐barrier system.     

Smart optics in ast

Smart optics means much more than adaptive optics on telescopes: these new technologies are changing the way space instruments are built and operated and are bringing new technologies into everyday life in the form of cheaper, lighter and more robust optical systems. Steve Welch, Peter Doel, Alan Greenaway and Gordon Love summarize the work of the Smart Optics Faraday Partnership in the UK.     

Exploring hydroclimatic change disparity via the Budyko framework

The Budyko framework characterizes landscape water cycles as a function of climate. We used this framework to identify regions with contrasting hydroclimatic change during the past 50 years in Sweden. This analysis revealed three distinct regions: the mountains, the forests, and the areas with agriculture. Each region responded markedly different to recent climate and anthropogenic changes, and within each region, we identified the most sensitive subregions. These results highlight the need for regional differentiation in climate change adaptation strategies to protect vulnerable ecosystems and freshwater resources. Further, the Budyko curve moved systematically towards its water and energy limits, indicating augmentation of the water cycle driven by changing vegetation, climate and human interactions. This finding challenges the steady state assumption of the Budyko curve and therefore its ability to predict future water cycles. Copyright © 2013 John Wiley & Sons, Ltd.     

The First Hop: Use of Beaufort Sea Deltas by Hatch-Year Semipalmated Sandpipers

River deltas along Alaska’s Beaufort Sea coast are used by hatch-year semipalmated sandpipers (Calidris pusilla) after leaving their terrestrial natal sites, but the drivers of their use of these stopover sites on the first “hop” of fall migration are unknown. We quantified sandpiper temporal distribution and abundance as related to food resources at three river deltas during the beginning of their fall migration (post-breeding period) to compare the habitat quality among these deltas. We conducted population counts, sampled invertebrates, and captured birds to collect blood samples from individuals for triglyceride and stable isotope analyses to determine fattening rates and diet. Patterns of sandpiper and invertebrate abundance were complex and varied among deltas and within seasons. River deltas were used by sandpipers from late July to late August, and peak sandpiper counts ranged from 1000 to 4000 individuals, of which 98% were hatch-year semipalmated sandpipers. Isotopic signatures from blood plasma samples indicated that birds switched from a diet of upland tundra to delta invertebrate taxa as the migration season progressed, suggesting a dependence on delta invertebrates. Despite differences in diet among deltas, we found no differences in fattening rates of juvenile sandpipers as indicated by triglyceride levels. The number of sandpipers was positively associated with abundance of Amphipoda and Oligochaeta at the Jago and Okpilak-Hulahula deltas; an isotopic mixing model indicated that sandpipers consumed Amphipoda and Oligochaeta at Jago, mostly Chironomidae at Okpilak-Hulahula and Spionidae at Canning. Regardless of the difference in sandpiper diets at the Beaufort Sea deltas, their similar fattening rates throughout the season indicate that all of these stopover sites provide a critical food resource for hatch-year sandpipers beginning their first migration.     

Flood seasonality across Scandinavia—Evidence of a shifting hydrograph?

Fluvial flood events have substantial impacts on humans, both socially and economically, as well as on ecosystems (e.g., hydroecology and pollutant transport). Concurrent with climate change, the seasonality of flooding in cold environments is expected to shift from a snowmelt‐dominated to a rainfall‐dominated flow regime. This would have profound impacts on water management strategies, that is, flood risk mitigation, drinking water supply, and hydro power. In addition, cold climate hydrological systems exhibit complex interactions with catchment properties and large‐scale climate fluctuations making the manifestation of changes difficult to detect and predict. Understanding a possible change in flood seasonality and defining related key drivers therefore is essential to mitigate risk and to keep management strategies viable under a changing climate. This study explores changes in flood seasonality across near‐natural catchments in Scandinavia using circular statistics and trend tests. Results indicate strong seasonality in flooding for snowmelt‐dominated catchments with a single peak occurring in spring and early summer (March through June), whereas flood peaks are more equally distributed throughout the year for catchments located close to the Atlantic coast and in the south of the study area. Flood seasonality has changed over the past century seen as decreasing trends in summer maximum daily flows and increasing winter and spring maximum daily flows with 5–35% of the catchments showing significant changes at the 5% significance level. Seasonal mean daily flows corroborate those findings with higher percentages (5–60%) of the catchments showing statistically significant changes. Alterations in annual flood occurrence also point towards a shift in flow regime from snowmelt‐dominated to rainfall‐dominated with consistent changes towards earlier timing of the flood peak (significant for 25% of the catchments). Regionally consistent patterns suggest a first‐order climate control as well as a local second‐order catchment control, which causes inter‐seasonal variability in the streamflow response.